1. Field of the Invention
The present invention relates in general to duplex phase stainless steels having austenite-ferrite duplex phase matrix and good resistance to both stress corrosion cracking and pitting, and suitable for use in the areas of heat exchangers using seawater as cooling water, tanks and pipes of desalination plants, FGD (Flue Gas Desulfurization) equipments fossil power plants, tubes and pipes of refineries and petrochemical plants, equipments of chemical plants and waste water disposal plants.
2. Description of the Prior art
It has been typically noted that stainless steels are special steels having excellent corrosion resistance in comparison with the other alloy steels. However, typical commercial stainless steels have no good resistance against both stress corrosion cracking (SCC) and crevice corrosion, such as pitting, so that the typical stainless steels can not be used as materials of equipments for the environments including high concentration of chloride ion. In this regard, titanium alloy or nickel-based super alloy instead of the typical stainless steels are used as the material of equipments for the environments including high concentration of chloride ion.
However, the titanium alloy and the nickel-based super alloy are not only limited in their production amounts but also very expensive in comparison with the typical stainless steels. In this regard, there have been continuous studies on the development of improved corrosion resistant stainless steel by controlling composition of alloy elements of the stainless steel.
For example, both AISI 316 (Sammi Specialty Steel Co. Ltd., Korea) produced by addition of 2-3% of Mo to austenitic stainless steel of AISI 304 and the austenitic stainless steel such as nitrogen-laden AISI 317 LNM (Creusot-Loire Industrie, France) being noted to have somewhat improved the corrosion resistance of the stainless steel. However, those stainless steels are also noted to have poor resistance against SCC in specified corrosion environments, such as chloride ion-containing solution under tensile stress. In an effort to overcome the problems of those stainless steels, duplex phase stainless steel having austenite-ferrite duplex phase matrix has been proposed.
However, the corrosion resistance of the duplex phase stainless steel will be reduced in the case of aging heat treatment of the stainless steel. In this regard, the corrosion resistance of the stainless steel goods can not help being reduced when the steel is heated such as by welding. Such reduction of corrosion resistance of the typical corrosion resistant stainless steel due to the aging heat treatment is caused by transformation of the ferrite phase of the duplex phase stainless steel into austenite II phase and sigma phase including large amount of chromium and molybdenum and having high hardness.
U.S. Pat. No. 4,500,351 discloses a cast duplex phase stainless steel which generates no pitting in anode polarization at temperatures of 50.degree. C.-78.degree. C. in 1 mole NaCl solution but generates crevice corrosion at 47.5.degree. C. in 10% FeCl.sub.3.6H.sub.2 O.